Recently, with the decrease in size and increase in performance of electronic apparatuses, it has been strongly demanded that multilayer wiring boards on which semiconductor chips of large scale integration (LSI) or the like can be mounted in a high density are supplied inexpensively in a large field of not only industrial application but also consumer application. In such a multilayer wiring board, wiring patterns of a plurality of layers formed in a fine wiring pitch must be interconnected electrically with high wiring reliability, and the thinning of the board is strongly demanded.
One conventional example satisfying such demands of the market is disclosed in Japanese Patent Unexamined Publication No. H06-268345 (Document 1). Document 1 discloses an inner via hole (IVH) connection method capable of interlayer-connecting any electrodes of a multilayer wiring board at any wiring pattern positions, namely discloses a resin multilayer wiring board with an any-layer IVH structure. The resin multilayer wiring board having the any-layer IVH structure is used as an alternative to the conventional multilayer wiring board where the metal plated conductor on the inner wall of a through hole mainly provides interlayer insulation. In the resin multilayer wiring board having the any-layer IVH structure, only required layers can be interconnected by filling a conductor into the via hole of the multilayer wiring board, and an inner via hole can be disposed just under a component land. Thus, decrease in board size and increase in mounting density can be achieved. Conductive paste is used for the electric connection in the inner via hole, so that stress on the via hole can be reduced, and electric connection stable for dimensional change by thermal shock or the like can be achieved.
The conventional resin multilayer wiring board having an any-layer IVH structure is manufactured in the steps shown in FIG. 3A through FIG. 3I, as disclosed in Document 1, for example.
Electrical insulating substrate 301 shown in FIG. 3A is a porous substrate having compression property, a substrate having a three-layer structure having adhesive layers on both sides of a core film, or a composite substrate of fiber and resin. Cover films 302 are stuck to both sides of electrical insulating substrate 301 by laminating as shown in FIG. 3A.
Then, in FIG. 3B, via holes 303 penetrating all of electrical insulating substrate 301 and cover films 302 are formed using a laser or the like.
In FIG. 3C, via holes 303 are filled with conductive paste 304. At this time, the cover films prevent the conductive paste from remaining on the electrical insulating substrate.
In FIG. 3D, cover films 302 on the both sides are peeled to expose electrical insulating substrate 301, and foil-like wiring materials 305 are laminated on both sides.
In FIG. 3E, wiring materials 305 are stuck to electrical insulating substrate 301 by heating and pressing. Electrical insulating substrate 301 has compression property, so that electrical insulating substrate 301 is contracted in a thickness direction by heating and pressing. In this heating and pressing step, conductive paste 304 is also compressed in the thickness direction. Metal fillers in the conductive paste are brought into contact with each other in a high density by the compression, so that wiring materials 305 are electrically connected to conductive paste 304. Here, the high density contact state means that many metal fillers are in contact with each other and the contact area of the metal fillers is large.
In FIG. 3F, double-sided wiring board 306 is finished by patterning wiring materials 305.
In FIG. 3G, electrical insulating substrate 307 filled with the conductive paste produced in the same steps shown in FIG. 3A through FIG. 3D is laminated on one surface of double-sided wiring board 306. At this time, electrical insulating substrate 307 is positioned by recognizing the position of the wiring pattern of previously formed double-sided wiring board 306. Wiring material 308 is laminated on the other surface of electrical insulating substrate 307. In forming via holes in electrical insulating substrate 307, laser machining data is corrected based on the measurement result of dimensional change in the surface direction of double-sided wiring board 306.
In FIG. 3H, wiring materials 308 are stuck to electrical insulating substrates 307 by heating and pressing. At this time, simultaneously, double-sided wiring board 306 is stuck to electrical insulating substrates 307. In this heating and pressing step of FIG. 3H, electrical insulating substrates 307 are contracted in the thickness direction, and conductive paste 309 is also compressed in the thickness direction, similarly to the step of FIG. 3E. Conductive paste 309 is brought into contact with wiring materials 308 and wiring 310 on the double-sided wiring board in a high density by the compression, and hence electric connection is achieved.
In FIG. 31, a multilayer wiring board is finished by patterning wiring materials 308 on the surfaces. Here, a four-layer board is shown as the multilayer wiring board; however, the number of layers of the multilayer wiring board is not limited to four. The number of layers can be increased by repeating the similar steps.
Another conventional example is disclosed in Japanese Patent Unexamined Publication No. 2000-77800, for example. This document discloses a structure where higher-density interlayer connection is achieved by decreasing the size of inner via holes and high reliability is achieved. FIG. 4 shows a manufacturing method and a structural characteristic of this conventional wiring board. In FIG. 4, the descriptions of steps similar to those in FIG. 3 are simplified. FIG. 4A through FIG. 41 are sectional views showing main steps of the manufacturing method of the conventional wiring board.
In FIG. 4A, electrical insulating adhesive 411 is formed on both surfaces of electrical insulating substrate 401, and cover films 402 are formed on both sides of the product.
In FIG. 4B, via holes 403 penetrating electrical insulating substrate 401 are formed. As electrical insulating substrate 401, similarly to the conventional example, a porous substrate having compression property, a substrate having a three-layer structure where adhesive layers are formed on both sides of a core film, or a composite substrate of fiber and resin is used. The via holes are formed by laser machining using a carbon dioxide laser, an excimer laser, or a YAG (yttrium aluminum garnet) laser.
In FIG. 4C, the via holes are filled with conductive paste 404.
In FIG. 4D, wiring transfer substrate 405 is formed of support substrate 406 and wiring 407 that is formed in a desired pattern on the support substrate. The wiring transfer substrate is generally formed by selectively etching only copper foil of composite foil where the copper foil is laminated on aluminum foil in a desired pattern. The formation of the copper foil on the aluminum foil is usually performed by electrolysis plating, and stress between aluminum and copper is extremely small. In other words, it is structured in such a manner that the dimensional change in the surface direction is small when the wiring pattern is formed by etching the copper foil.
Then, cover films 402 are peeled from the surfaces of electrical insulating substrate 401. Wiring transfer substrate 405 is disposed on one surface of electrical insulating substrate 401 having electrical insulating adhesive 411 on its both surfaces, and wiring material 408 is disposed on the other surface, as shown in FIG. 4D.
In FIG. 4A through FIG. 4D, simply, the forming step of the electrical insulating substrate is firstly described; however, actually, wiring transfer substrate 405 may be firstly formed. In this case, the previously formed wiring pattern is positionally recognized, laser machining data is corrected in response to the positions of wiring 407, and via machining can be performed.
Then, in FIG. 4E, wiring transfer substrate 405, electrical insulating substrate 401, and wiring material 408 are stuck to each other by heating and pressing. At this time, the wiring on wiring transfer substrate 405 is buried in electrical insulating substrate 401. Conductive paste 404 filled into via holes 403 is effectively compressed by burying of wiring 407, metal fillers in conductive paste 404 are brought into contact with each other in a high density, conductive paste 404 is electrically connected to wiring 407, and conductive paste 404 is electrically connected to wiring material 408. Then, in FIG. 4F, wiring material 408 on the surface is patterned by etching to form wiring board 409 having two-layer wiring 407.
Then, in FIG. 4G, two-layer wiring boards 409 are positioned and laminated on both sides of electrical insulating substrate 410 filled with the conductive paste. Here, electrical insulating substrate 410 is manufactured in the manufacturing method similar to the above-mentioned manufacturing method of electrical insulating substrate 401. In each figure, the laminated wiring transfer substrate is simply illustrated in the same wiring pattern; however, generally, a different wiring pattern is used.
Next, in FIG. 4H, the electrical insulating substrates are stuck to each other by heating and pressing. Then, in FIG. 4I, support substrates 406 are removed from the surfaces to finish a multilayer wiring board. Here, the removing method of support substrates 406 depends on an employed material. When metal material is employed as support substrates 406, a removing method using dissolution by chemicals is excellent in productivity. When resin sheets are employed as the support substrates, they are generally peeled mechanically.
A four-layer board is taken as an example of the multilayer wiring board; however, the number of layers of the multilayer wiring board is not limited to four. The number of layers may be increased in similar steps. In the conventional manufacturing method, the conductive paste is compressed in the thickness direction due to compression property of the electrical insulating substrate, or is effectively compressed by burying the wiring into the electrical insulating substrate. Thus, metal fillers in the conductive paste are brought into contact with each other in the high density, and the wiring material is electrically connected to the conductive paste.
However, recently, it has been strongly demanded that the multilayer wiring board is thinned, and, when a thin electrical insulating substrate is employed for satisfying this demand, little compression property is demonstrated. Therefore, a phenomenon that electric connection resistance between the wiring material and the conductive paste varies on the high side, or a phenomenon in which the via resistance stability deteriorates in a reliability test can be observed. In a general manufacturing method using no wiring transfer method, when the number of layers is increased using thin electrical insulating substrates, no wiring is buried in both surfaces of one of the electrical insulating substrates. Therefore, the compressing effect of the conductive paste by burying the wiring into the electrical insulating substrate cannot be expected.
In other words, the conventional manufacturing method has the following problems when a multilayer wiring board having an any-layer IVH structure is manufactured using thin electrical insulating substrates. As one problem, it is difficult to effectively compress the conductive paste and hence the electric connection resistance between the wiring material and the conductive paste varies on the high side. As another problem, via resistance stability deteriorates in the reliability test. Therefore, it is difficult to thin the multilayer wiring board having an any-layer IVH structure.